Population immunity levels provide an insight into the risk that the population faces from a particular set of infectious diseases. Measuring immunity, however, is complex. What is generally measured instead, is compliance with vaccination schedules. However, there is a difference between vaccination and immunity. This difference is important because children may develop immunity to an infectious disease by experiencing and recovering from or being exposed to an illness, rather than from being vaccinated. Also vaccines may not provide immunity to a child if they are inappropriately stored, handled, or administered. Moreover, the effectiveness of vaccines in inducing immunity in a child varies and no vaccine is 100% effective (National Health and Medical Research Council (NHMRC) 2001). Both the Australian Bureau of Statistics (ABS) and the Australian Childhood Immunisation Register (ACIR) measure and report on compliance with vaccination schedules among Australian children, rather than the levels of immunity within this population (see glossary for definitions of immunisation and vaccination).

Current Government policy (see 1.1 below) provides incentives, such as direct links to family payments, to encourage widespread vaccination of Australian children. The basis for this policy is that universal immunisation of children minimises the spread of infectious disease and reduces death and serious illness among Australian children. The risks associated with immunisation are considered to be far less than the risks of disease if the population was not immunised (NHMRC 2001). In 2001 children aged 0 to 6 years were required to be vaccinated against Diphtheria, Tetanus, Pertussis, Polio, Measles, Mumps, Rubella, Haemophilus Influenzae Type b (Hib) and Hepatitis B, in accordance with the Australian Standard Vaccination Schedule.

1.1 Immunise Australia Program

The Immunise Australia program is a joint Commonwealth-State/Territory Government program which aims to increase national childhood immunisation rates so as to reduce the incidence of vaccine preventable diseases in the Australian community. The NHMRC established the following immunisation coverage targets for the year 2000:

greater than 90% immunisation coverage of children at 2 years of age for all diseases specified in the schedule;

near universal immunisation coverage at school entry; and

near universal coverage of girls and boys under 17 years of age for Measles, Mumps and Rubella (MMR).

The Immunise Australia Program builds on initiatives developed under the National Childhood Immunisation Program, which includes the provision of free vaccines to all providers; the establishment of the Australian Childhood Immunisation Register; provider and community education programs; and a national adverse events reporting scheme.

1.2 ABS vaccination data

The ABS collected childhood immunisation data in 1989-90, 1995 and 2001. Data collected in 1989-90 and 2001 were part of the National Health Survey (NHS), while 1995 data were collected as part of the Children's Immunisation and Health Screening Survey. In all three surveys adults were interviewed in the home and asked questions to determine the status of children aged six years or less relative to national vaccination schedules applicable at the time of interview. The 2001 NHS provides for comparisons between Indigenous and non-Indigenous children for non-remote areas of Australia (i.e major cities and inner and outer regional Australia) and coverage estimates for all children at national and State/ACT level (the sample size in the NT was insufficient to provide reliable estimates) . Given the development of the ACIR's reporting capabilities, the ABS does not plan to collect data on scheduled vaccinations for all Australian children aged 0 to six yearsin the 2004-5 NHS, although some data for Indigenous children are planned to be collected. In addition, statistics on vaccines not currently scheduled, such as for Varicella (chicken pox), are also being considered for inclusion in the survey.

1.3 ACIR vaccination data

The Australian Childhood Immunisation Register (ACIR) was developed in response to a decline in childhood immunisation coverage in Australia and an increase in preventable childhood diseases. The ACIR commenced on 1 January 1996 and records details of vaccinations given to children under the age of seven who live in Australia.The register is administered by the Health Insurance Commission (HIC) on behalf of the Commonwealth Department of Health and Ageing (DoHA) and statistics are published quarterly. In addition to national and State/Territory statistics, some local area data are available. Comparisons between Indigenous and non-Indigenous children are not currently available.

The 1989-90 National Health Survey was conducted by the ABS during the twelve months October 1989 to September 1990 at approximately 22,200 private and special dwellings (e.g. hotels, motels, boarding houses) selected throughout Australia. Hospitals and nursing homes were excluded from the sample. Information about children's immunisation was obtained from an adult member of the household reporting on behalf of the child. Respondents were encouraged to consult immunisation cards or records to assist them in providing the information. However, records were not consulted in 55 per cent of cases, and this may have affected the quality of the information reported.

2.2 The 1995 Children's Immunisation and Health Screening Survey

The 1995 Children's Immunisation and Health Screening Survey was conducted as part of the regular monthly population survey, which is based on a national multi-stage area sample of about 30,000 private dwellings covering about one-half of 1 per cent of the population of Australia. Around 14,800 children aged 0 to 14 years were included in the survey and information on their compliance with vaccination schedules was provided by parents or guardians. Interviews were conducted over a period of two weeks in April 1995.

Where possible the information was obtained from the child's mother or a female guardian, otherwise the father or a male guardian responded. Information was provided by the mother/female guardian for 88.5% of children covered by the survey. If a parent or guardian of the child was not available, another adult who was identified as being responsible for the child was interviewed.

In an effort to improve the reliability of the information provided, respondents were encouraged to refer to children's personal health records during interview whenever possible. Records were consulted for 61% of children aged from 3 months to 6 years covered by the survey.

2.3 The 2001 National Health Survey

The 2001 NHS was conducted during the 10 month period February to November 2001. The survey was conducted at approximately 17,900 private dwellings selected throughout non-sparsely settled areas of Australia. The sample design ensured that within each State or Territory each person had an equal chance of selection. Information was obtained about one adult, all children aged 0 to 6 years, and one child aged 7 to 17 years in each selected household. Almost 27,000 persons (including just under 9,000 children aged 0 to 17 years) fully responded to the survey.

Information about childhood immunisation was collected for all children aged 0 to 6 years. A nominated adult reported on behalf of children. Respondents were encouraged to refer to immunisation records (e.g. Baby Book) or other information to assist them to reliably report on their child's vaccination status. Data were obtained about the type of record consulted (if any) and the reported immunisation status of the child. Statistics were also collected on factors influencing the decision to immunise and on reasons for not immunising children.

Overall, 82% of respondents had some type of immunisation record they referred to. However, the use of records did not necessarily ensure accurate reporting since some records were incomplete or poorly completed. Interviewers were instructed that if there was at least a date in the relevant section of the record then it should be considered that the child had received that particular vaccine.

The age group covered in the survey spanned three versions of the recommended childhood immunisation schedule (1996, 1998 and 2000). Taking account of all schedule changes in the survey is complex for both data processing and for users when interpreting the data. Notable changes and factors that may have affected reporting include:

a national campaign in 1998 to vaccinate children with the last MMR booster at ages over 4 years rather than at ages 10 to 16 years as previously. This effectively constituted an amendment to the schedule in that children were then required to have received two MMR vaccines by the time they were 6 years old.

HepB was included in the recommended schedule for the first time in 2000. This change meant that children born before 1 May 2000 would be classified as fully vaccinated if they received all other vaccines except HepB, but that children born on 1 May 2000 or later would be considered fully vaccinated only if they had received HepB vaccinations in addition to the others listed in the schedule.

A total of 3,200 Aboriginal and Torres Strait Islander adults and children from across Australia were included in a supplementary Indigenous sample to the 2001 NHS which was conducted throughout Australia from June to November 2001. The supplementary survey provides for comparisons of immunisation coverage between Indigenous and non-Indigenous children aged less than seven years from non-remote Australia (i.e. major cities and inner and outer regional Australia) (refer glossary for definition of inner and outer regional Australia).

2.4 ACIR collection methods

The ACIR is the national immunisation database. Health professionals and State/Territories use the ACIR to monitor immunisation coverage levels and service delivery. The HIC collects immunisation data to provide comprehensive information on the immunisation status of all children under seven years of age living in Australia.

All children under the age of seven years enrolled in Medicare are included in the ACIR. A sub-set of age cohorts are selected for assessment of their vaccination status as part of ACIR statistical reporting. The three age cohorts assessed in 2002 were 12 to less than 15 months, 24 to less than 27 months, and 72 to less than 75 months.

Changes to a child's details, such as a change of address on the Medicare enrolment database are routinely updated on the ACIR. Additionally, where children are not enrolled with Medicare, they can be added to the ACIR when details of an immunisation are supplied by an immunisation provider. Data are collected from medical practitioners and other immunisation providers, such as community health clinics. The details of a child's immunisation status can be reported on a manual encounter form, through electronic data interchange, or by sending details to a secure internet site administered by the HIC.

3. DERIVATION OF VACCINATION COVERAGE STATISTICS

3.1 How were ABS vaccination coverage statistics derived?

ABS derivations of vaccination coverage were designed to allow up to one month after the age at which the vaccination was due, according to the appropriate schedule that child was following. For example, under the 2000 schedule, a child should receive a Diphtheria/Tetanus/Pertussis (DTP) vaccination at 2, 4 and 6 months of age. A child aged 5 months (or more) would not be regarded as fully immunised unless they had at least 2 DTP vaccinations, but a child aged less than 5 months (e.g. 4 months and 27 days) would be deemed to be fully vaccinated with at least 1 DTP vaccination.

The scope of ABS childhood immunisation data is every child aged less than 7 years. For the purposes of deriving children's immunisation status, exact age was derived from date of interview and date of birth information. In 2001 children less than three months of age were deemed to be fully immunised, unless born after the 1st of May 2001, in which case they were required to have received a HepB vaccine at birth or shortly after. All vaccinations required by a child of a certain age were in scope when the estimate for full immunisation was derived.

3.2 How were ACIR immunisation statistics derived?

In regard to the methodology used to determine if a child on the ACIR is fully immunised, Edward, Sam and Mead (1997) state:"To be considered fully immunised a child should have completed the number and type of vaccinations listed in the National Health and Medical Research Council (NHMRC) standard childhood vaccination schedule. Thus, at 1 year of age, a child should have completed the primary series with three vaccinations against diphtheria, tetanus and pertussis (DTP or CDT plus monovalent pertussis), three poliomyelitis (OPV or IPV) and either two or three Hib vaccinations (if the vaccine used was PedvaxHIB or HibTITER, respectively). At 2 years of age a child should have completed the primary series as well as MMR (due at 12 months), Hib (PedvaxHIB at 12 months or HibTITER at 18 months) and DTP (due at 18 months)."

The methods as outlined above apply to the age cohorts 12 to less than 15 months and 24 to less than 27 months. Estimates of fully immunised children in the oldest age cohort (72 to less than 75 months) differed, in that the Hib vaccine was not included when this estimate was derived, although it is in scope for this age cohort. Table 1 summarises the main differences between ACIR estimates available from the HIC website (www.hic.gov.au) and ABS survey estimates from the 2001 NHS.

Table 1- Main differences in calculating the immunisation status of Australian children - ABS and ACIR estimates

ABS 2001 NHS

ACIR 2002

Age

All children aged 0-6 years

Separate age cohorts (12-14 months, 24-26 months and 72-74 months)

Type

All vaccinations on the applicable schedule to determine immunisation status

DTP, MMR, HepB, Hib and Polio to determine immunisation status for two youngest age cohorts. Hib not assessed for 72-74 month cohort, although in scope based on the Australian Standard Vaccination Schedule.

Time

Given one month leeway for receiving vaccination (e.g. if needed vaccination at age 2 months given to 3 months before being deemed to be not fully immunised)

Given more than one month leeway for receiving vaccination relative to schedule (period of time varies with age cohort).

Dose

Number of boosters used to derive status

If an older child has a recorded booster then it is assumed that all other earlier boosters have been given (see "third dose assumption")

The "Third Dose Assumption" concerns the way ACIR estimates are derived for some children. In brief, if a child's record indicated that the child had received the last vaccine due in each sequence then it was assumed that earlier vaccinations in the sequence had been given.For example, if a record of a child was provided and indicated that the third DTP injection was given, it was assumed that the first and second injections of DTP had also been administered.

Table 2 shows how ACIR age cohorts and the age groups used by the ABS relate in practice to key immunisation milestones, as defined by the Australian Standard Vaccination Schedule.

(a) Combined dephtheria-tetanus vaccine (CDT) booster at 4-5 years (or prior to school entry) was replaced by diphtheria, tetanus and pertussis vaccine (DTP), August 1994.(b) Children aged 0-12 months have been excluded from estimates for Measles-Mumps-Rubella (MMR).(c) Vaccination against Rubella at one year of age was introduced from 1991.(d) Requirement for vaccination against Hib was introduced into the recommended schedule in April 1993.(e) Vaccination against Hepatitis B was introduced into the recommended schedule in May 2000.

Due to changes in the National Health and Medical Research Council's (NHMRC) recommended vaccination schedule for children, the results of ABS surveys presented in Table 3 from 1989-90, 1995 and 2001 are not directly comparable. This is because each addition of a new vaccine over time increases the number of criteria that need to be met for a child to be considered fully vaccinated. Moreover, in 2001, the concept used to determine if a child was fully vaccinated was compliance with the schedule started, and up to three different schedules were applicable to the population of interest (children aged 0 to six years). In contrast, in 1989-90 and 1995, only compliance with specific schedules was reported (i.e. compliance with the 1986, 1991 and 1994 schedules). Note that children less than 3 months in age were excluded from estimates of coverage published in April 1995 Children's Immunisation Australia (4352.0), but are included as "fully immunised" in Table 3.

In 1995 the proportion of fully vaccinated children based on the 1994 vaccination schedule was around one-third of children aged 0 to six years. However, the timing of changes in the NHMRC's recommended schedule, particularly the introduction of the Hib vaccine in 1993, was a major factor contributing to this result. When measured against the previous (1991) vaccination schedule, the proportion of children reported as fully vaccinated in 1995 was 53% of children. In all three surveys, rates of vaccination against particular diseases were much higher than rates of children determined as fully vaccinated against all applicable diseases. This difference reflects the more numerous criteria for being fully vaccinated against all diseases, compared to full vaccination against a particular disease.

An important aspect of vaccination statistics collected as part of the 2001 National Health Survey is that they can be analysed using other contextual information collected as part of the survey (e.g. socioeconomic and demographic variables). Appendix D provides an example of this type of analysis and matches characteristics, such as family type and labour force status, with the percentage of children who were fully immunised in these households.

4.2 ACIR estimates of vaccination coverage for 2002

Tables 4 presents the results for vaccination coverage derived from the ACIR for Australia as disseminated in a quarterly report available from the HIC website (www.hic.gov.au). Also available from the ACIR are State/Territory estimates and some local area data, in addition to many other reports designed to support and inform vaccination programs and providers. Differences in methods, outlined earlier, account for the estimates in Table 4 being higher than ABS survey estimates. Estimates of fully immunised children were the most affected by differences in methods and care must be taken in using statistics from those different data sets. Results from the ACIR were first disseminated as preliminary estimates of vaccination coverage for one year old cohorts born in 1996 (O'Brien, Sam and Mead 1997).

Table 4. ACIR estimates of vaccination coverage for Australian children 2002(a)

Age Cohort

% DTP

% OPV

%HIB

% Hep B

%MMR

% Fully Immunised(b)

12-14 months

92.7

92.6

94.9

95.1

not assessed

91.7(c)

24-26 months

91.4

94.8

93.9

not applicable

94.1

89.4(d)

72-74 months

84.5

84.7

not assessed

not applicable

83.7

82.2(e)

(a) Date of processing as at 31 December 2002.(b) Percentage fully immunised equals the number of children vaccinated divided by the number of children in the register.(c) Only vaccines administered before 12 months are included in the coverage calculation.(d) Only vaccines administered before 24 months are included in the coverage calculation.(e) Only vaccines administered before 72 months are included in the coverage calculation and Hib was not assessed.

The effect of allowing more time for a child to be considered vaccinated relative to the Australian Standard Vaccination Schedule (one of the differences between ABS and ACIR coverage estimates) is demonstrated in Table 5. This table compares data from the 2001 NHS, processed to match the leeway times on the ACIR, with ABS estimates as they are usually derived (i.e. one month leeway for a scheduled vaccine to be administered). There are no effects for DTP or Polio, but some differences for Hib and MMR.

When vaccination coverage is derived from ABS data using similar leeway times to the ACIR then the estimates are broadly consistent and many of the estimates are the same. Remaining differences between ACIR and ABS vaccination coverage estimates (see Table 4 for ACIR estimates) reflect fundamental differences in the way the statistics were collected. ABS estimates are self-reported data from household surveys and rely on the ability of parents and guardians to report reliably on their child's vaccination records, preferably by referring to written records at interview, and no earlier vaccinations were assumed as given. ABS estimates are also subject to confidence intervals as they are estimates from sample surveys. ACIR estimates assume, in some instances, that earlier vaccinations were given. They also rely on the quality of information sent in by providers of immunisation services, an issue that has caused difficulties with quality in the earlier years of the Register (Human Capital Alliance 2000). In response to the recommendations of the 2000 evaluation, a data quality plan has been introduced. It should be noted, however, that the main problem detected with ACIR records concerned under-reporting of vaccination coverage (i.e. children were reported as not fully immunised, when a check of their status determined that they were in fact fully immunised) (Human Capital Alliance 2000).

GLOSSARY

CDT - trade name for diphtheria-tetanus vaccine made by CSL for use in children.

DTP - a vaccine that protects against diphtheria, tetanus and pertussis (whooping cough).

Hib - Haemophilus Influenzae type b - a bacterium that causes meningitis and other serious infections in young children.

Inner Regional Australia - Refers to areas with an average Accessibility/Remoteness Index of Australia (ARIA) value of greater than 0.2 and less than or equal to 2.4 (i.e. areas that are more remote than major cities and less remote than Outer Regional Australia, see below).

Outer Regional Australia - Refers to areas with an average Accessibility/Remoteness Index of Australia (ARIA) value of greater than 2.4 and less than or equal to 5.92 (i.e. areas that are more remote than major cities and Inner Regional Australia and more accessible than Remote and Very Remote Australia).

Immunisation - the process of inducing immunity to an infectious agent by administering a vaccine.

Vaccination - the administration of a vaccine; if vaccination is successful, it results in immunity.

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